Transient spectroscopy has been used to systemetically study the photo-induced dissociation of carbon monoxide from hemoglobin and myoglobin. The data provide a detailed description of the pathway for ligand dissociation. They show three distinct structural relaxations in the unliganded protein. The relationship between these structural changes and ligand binding has been probed in two new ways in the past year. (1) A study of hybrid cobalt-iron hemoglobin in which only the Fe heme bind ligand: the use of hybrid hemoglobins simplifies the analysis of the data and allows us to use the Co hemes as reporter groups to determine how rapidly structural changes initiated by photolysis of CO from the Fe hemes are propogated to the Co hemes. (2) An examination of the dependence of the rebinding kinetics on CO pressure. The photolysis studies have also been extended to examine the dissociation of oxygen from hemoglobin. This important physiological reaction nearly all of the functional cooperativity results from structurally-induced changes in the overall dissociation rate. The results show a similar set of processes to those observed in CO hemoglobin, but a substantial differences in the rates for geminate recombination.